In recent years, transmedia has come into the spotlight among those creating and using media and technology for children. We believe that transmedia has the potential to be a valuable tool for expanded learning that addresses some of the challenges facing children growing up in the digital age. Produced by the USC Annenberg Innovation Lab and the Joan Ganz Cooney Center, this paper provides a much-needed guidebook to transmedia in the lives of children age 5-11 and its applications to storytelling, play, and learning. Building off of a review of the existing popular and scholarly literature about transmedia and children, this report identifies key links between transmedia and learning, highlights key characteristics of transmedia play, and presents core principles for and extended case studies of meaningful transmedia play experiences. The authors hope that T is for Transmedia will incite conversation among diverse stakeholders including educators, entertainment industry executives, creative artists, academic scholars, policy makers, and others interested in the future of children's learning through transmedia.